1. Field of the Invention
The present invention relates to a transmission power control method for controlling the transmission power of packet signals to be transmitted from a mobile station via an upstream radio channel, in a radio communication system for allowing radio communications between a base station and a plurality of mobile stations via code division multiple access (CDMA) radio channels. The present invention also relates to a radio communication system, a base station and a mobile station which can be used in the above transmission power control method.
2. Description of the Related Art
The conventional radio communication system for transmitting signals via CDMA radio channels differs from the radio communication systems for transmitting signals via time division multiple access (TDMA) radio channels or frequency division multiple access (FDMA) radio channels in the following way. In the conventional radio communication system, a receiver (for example, a base station) can decode despread signals which satisfy a required SIR (Signal to Interference Power Ratio) even though a transmitter (for example, a mobile station) transmits a plurality of signals at the same time.
In this case, the receiver observes signals other than required signals as interference signals, so that the required signals do not satisfy the required SIR when the received power of the signals other than the required signals is larger than the received power of the required signals. Therefore, a “near-far effect”, in that the receiver can not decode the required signals, occurs.
To resolve the near-far effect., the transmitter is configured to control the transmission power of signals in the CDMA radio communication system.
Generally speaking, a first control method for controlling the transmission power of signals in each transmitter, so as to keep the received power of the signals in the receiver constant (predetermined received power), and a second control method for controlling the transmission power, so as to keep an SIR of the signals transmitted via each radio channel constant, are adopted as the control method of the transmission power of the above signals.
Normally, the CDMA radio communication system allows the transmitter to transmit signals with the minimum possible transmission power when the second control method is adopted, so as to improve the efficiency of usage of the available frequency, when compared to the first control method. Therefore, W-CDMA radio communication systems adopt the second control method.
However, the radio communication system adopting the second control method has a problem in that the transmitter determines the transmission power of the signals based on the interference power observed in the receiver, so that the interference power when the transmission power was determined often differs from the interference power during actual transmission of packet signals which are burst-type traffic. Thus, the required signals do not satisfy the required SIR in the receiver and there is a high possibility of the occurrence of communication errors when signals such as the packet signals are transmitted and the interference power observed in the receiver changes during a short period.
In other words, the second control method has a problem in that there is a possibility of causing lower throughput than a method without transmission power control in the transmitter.
To resolve the above problem, a method for determining transmission power with a predetermined margin can be considered. However, this method has a problem in that the determined transmission power becomes greater than necessary, so as to cause a reduction in channel-efficiency.
The method has a problem in that the required SIR and the predetermined margin are controlled based on the ratio (dB) between the received power and the interference power in the receiver, thus making it susceptible to burst-type interference signals (for example, packet signals) when the interference power is small.
On the other hand, the first control method has a problem in that the receiver can receive required signals (packet signals) when the interference power is smaller than a predetermined power, but the receiver can not receive required signals (packet signals) at all, when the interference power is larger than the predetermined power.
In other words, the radio communication system adopting the first control method has a problem in that the throughput characteristic is degraded abruptly in high traffic situations, differing from the radio communication system adopting the second control method which allows an increase in transmission power in the transmitters near the receiver, so as to satisfy the required SIR and prevent the abrupt degradation of throughput in high traffic situations.
The radio communication system adopting the first control method has a problem in that the transmitter can not transmit the required signals (packet signals), when the received power in the receiver does not satisfy the predetermined received power even though the receiver can receive the required signals (packet signals) correctly because the interference power in the receiver is small and satisfies the required SIR.
Therefore there is a problem in that the radio communication system adopting the first control method has a smaller communication area than the radio communication system adopting the second control method.
The radio communication system adopting the first and second control method has a problem in that the wasted transmission of the required signals (packet signals) in a transmitter which is far away from the receiver, and which does not satisfy the predetermined power and the required SIR, increases interference.
FIG. 1A is a graph showing a relationship between “traffic” and “throughput” in the radio communication system adopting the conventional first and second control method. FIG. 1B is a graph showing a relationship between “distance between a mobile station (a transmitter) and a base station (a receiver)” and an “average transmitter power in the mobile station” in the radio communication system adopting the conventional first and second control method.
As shown in FIG. 1A, the radio communication system adopting the first control method is characterized in that the throughput is high in low traffic situations, but the throughput is reduced abruptly in high traffic situations (referring to 801a).
As shown in FIG. 1A, the radio communication system adopting the second control method is characterized in that the throughput is lower than the radio communication system adopting the second control method in low traffic situations, but the throughput is not reduced abruptly in high traffic situations (referring to 801b).
As shown in FIG. 1B, the radio communication system adopting the first control method is characterized in that the average transmitter power in the plurality of mobile stations increases abruptly when the distance between the mobile station and the base station is longer than a predetermined distance (referring to 802a).
As shown in FIG. 1B, the radio communication system adopting the second control method is characterized in that the average transmitter power in the plurality of mobile stations increases gradually as the mobile stations recede from the base station (referring to 802b).